Reflow soldering is widely used to surface mount electronic components on a PC board or the like. Solder pastes are applied, and printed on the PC board, and components are then mounted on the soldering paste on the PC board. These elements are then heated in a reflow oven to melt the soldering paste to bond the electronic component on the PC board.
The soldering paste used in the reflow soldering contains a mixture of a flux and a solder powder. The flux includes a thixo agent, an activator, and a solvent mixed at an appropriate ratio with a rosin resin or a derivative of the rosin resin serving as the base material thereof. The solder power may be an alloy power based on an Sn—Pb system, an Sn—Ag system, an Sn—Cu system, an Sn—Ag—Cu system, an Sn—Bi system, an Sn—Sb system, an Sn—In system, an Sn—Zn system, or the like. The noticeable trend is that a solder powder containing no lead has been used in view of environment problems for recent years.
Since the solder powder and the activator in the flux are in a mixed state in these soldering pastes, the solder powder and the activator may react with each other during storage, causing the soldering paste to change in viscosity. Quality degradation takes place due to deteriorations with age, such as “skinned surface,” and “rough and crumbling.” “Skinned surface” refers to the state that the surface of the soldering paste is solidified to form a film. “Rough and crumbling” refers to the phenomenon that the entire paste loses viscosity.
The solder past suffering from deteriorations with age, such as a viscosity transition, “skinned surface,” and “rough and crumbling” during storage leads to a printing failure on a PC board, and an extreme degradation in wettability of the solder powder. Solder joint failures thus result. Several hours to several tens of hours may occasionally elapse between the printing of the soldering paste on the PC board and the reflow operation. If the deteriorations with age such as a viscosity transition, “skinned surface” and “rough and crumbling” take place during that period, wettability deteriorates. A joint failure thus results between the PC board and the electronic component. A variety of techniques have been proposed to overcome such problems.